As I understand it, his performance must be seen by all the members of the Tony nominating committee by April 27. So did the production scramble to get as many to the opening night as possible? Or will he have to perform all week to get those folks in? Then, assuming a nomination (safe assumption, I think), he'll have to be in enough performances for the 800+ voters to see him. That's quite a bit of strain on a damaged ACL.
(This is all assuming that this is about a Tony nomination for Karl and not just about getting the production through its opening night with the leading man in attendance.)
Above all, best wishes for a full recovery to the talented Mr. Karl, who would already have a Tony for his performance in "On the Twentieth Century" if it were up to me.